Estimation of cardinal temperatures and determination of the effects of tempera-ture and osmotic potential on safflower (Carthamus tinctorius L.) germination

Introduction Safflower (Carthamus tinctoriusL.) is annual and oil crop belong to Aster-aceae family that planted all over the world (Isigigure et al., 1995). Seed germination is a complex physiological process and one of the most important part of plant life cycle that affected by environmental factors. Temperature and moisture are the most important of these factors. Seed planting in optimum temperature resulted in maximum germination, uniformity and seedling establishment. Knowing minimum, maximum and optimum tem-perature of seed germination is necessary to appointment suitable planting date. In the oth-er way, environmental factors for seeding are not always favorable for seed germination. Osmotic stress is the most important factor that reduces growth and crop yield. Most re-ports showed that decline of osmotic potential cause a change cardinal temperature of seed germination. Hence the aim of this study is determination of cardinal temperature of safflower and the effects of different levels of temperature and osmotic potential on seed germination characteristics. Materials and methods The experiment was conducted as factorial in a completely ran-domized design with four replications, in agronomy laboratory, agricultural college, the University of Guilan in 2015. Experimental factors were including temperature (5, 10, 15, 20, 25, 30 and 35 ̊C) and osmotic potential (0, -0.2, -0.4, -0.6 and -0.8 MPa). Different osmotic potential prepared based on Michael and Kaufman (1973) method using with PEG 6000 and double distilled water. Germination percentage, germination rate and vigor index was calculated in the end of germination period (14 days). To estimation the cardinal tem-perature and needed biological hours for germination, five nonlinear regression models (polynomial quadratic, segmented, beta, dent like and curvelinear) were used. RMSe, R2, r and coefficient regression (a, b) related to amount of observed and predicted germination rate were used for finding the best model. Choosing the best model was conducted in two stages. At First, a model selected that amounts of parameters a, b were not significant dif-ferences with zero and one respectively. In the second stage, the model that had maximum R2 and minimum RMSe introduced as the selected model. Results The results showed that, minimum (%4) and maximum (%92) germination per-centage were observed in interactions of -0.8 MPa with 30 ̊C and 15 ̊C with control (dis-tilled water) respectively. Overall, the seed germination rate decreased with declining os-motic potential. The temperature levels of 25 and 35 ̊C had maximum and minimum ger-mination rate in all osmotic potential levels respectively. The treatment of osmotic poten-tial in comparison to the temperature on vigor index had the worse effect, nonetheless in all temperature levels reducing osmotic potential, was showed significant decrease. Based on model parameters measurement, only segmented and quadratic models had suitable fitness. Although segmented model had better fitness than quadratic model for safflower germination rate to temperature response. The preferable regression model estimated min-imum seed germination temperature (2.2 to 4.2 ̊C) optimum temperature (23.12 to 23.98 ̊ C) and maximum temperature (42.2 to 43.89 ̊ C). Decreasing in osmotic potential from control until -0.8 MPa causes decreasing minimum temperature in both models. The opti-mum temperature in -0.8 MPa osmotic potential increased nearly one degree in segmented model but in quadratic model this temperature is declined 3.25 ̊C degree. Also the effect of decreasing osmotic potential on maximum temperature was much observable than its effects on minimum and optimum temperature. So that maximum temperature in -0.8 MPa osmotic potential comparison control (zero MPa) in segmented model 9 ̊C and in quadratic model 6.3 ̊C was decreased. Overall, safflower seeds, protected their germination parame-ters suitably in sub optimal temperature in comparison with supra optimal temperature (in the same osmotic potential levels). Extreme decreasing of these parameters in high tem-perature (35 ̊C) expression the susceptibility of this crop to high temperature. Conclusions Expect preferable regression models and estimated parameters (minimum, maximum and optimum temperature) in preparation and assessment of seedling appear-ance models in field and determination of planting date in every area are used. Safflower is a tolerant plant to drought stress under suboptimal temperature, but is very sensitive at higher than 30̊C temperature. Therefore, it is a good candidate to be planted in suitable planting date even relatively low soil moisture condition.